The subject matter relates generally to power controllers having at least one of an inrush limiting circuit and/or an inductive kickback limiting circuit.
A power controller (e.g., solid state relay) may be required to operate switches that electrically power a load, such as a capacitive load or an inductive load. The load may appear as a short circuit through a switch when suddenly energized such that a relatively large current flows through the switch. This large amount of current (or inrush current) can potentially damage the switch and/or other circuitry of the power controller. When the load is inductive and suddenly turned off, a large kickback voltage may occur that can damage the switch or possibly other components of the power controller.
To address the inrush current problem, conventional power controllers often include an inrush current limiting circuit. For example, one or more resistors may be placed in series with the load. As another example, the power controller may be configured to have a first path (or branch) that is in parallel with a second path (or branch) prior to the load. Each of the first and second paths includes a respective switch that permits current to flow therethrough. When the power controller is activated (e.g., turned on), the switch of the first path is open and the switch of the second path is closed such that current flows through the second path. The second path functions to pre-charge the load and is typically activated for a predetermined time period after the power controller is activated. The second path is configured to pre-charge the load at a relatively slow rate during the predetermined time period. After the predetermined time period elapses, the second path is automatically deactivated and the first path is automatically activated. The first path may provide a greater performance or allow a higher current than the second path.
The predetermined time period in which the load is pre-charged is based on one or more factors, such as the configuration and application of the power controller. The predetermined time period is a fixed time period, regardless of the state of charge of the load when the power controller is activated.
Although the second paths described above may effectively limit the inrush current, there are some disadvantages to power controllers having them. For example, if the load is already fully charged or partially charged (e.g., from a previous activation), then it is not necessary to activate the second path for the entire time period. Consequently, the time until the first path is activated is delayed. This is generally not desirable, because the first path is more efficient than the second path. Additionally, the second path must be configured to withstand the repeated cycles of activation in which the current flows through the second path for the entire time period. As such, the second path must be larger and/or more costly than circuits that are used less often or that are activated for shorter time periods. In some cases, the second path includes a pulse-rated resistor that is configured for short but high-amplitude pulses of inrush current. This resistor, however, may have a lower dissipation rate than other resistors. If the second path is frequently activated, the second path can experience higher temperatures that may damage the power controller or the switching circuitry.
To address the kickback voltage problem, the switching circuits often include a transient voltage surge suppressor (TVSS) to absorb the large impulse voltage. The TVSS may be, for example, a Zener diode or an metal oxide varistor (MOV). TVSSs have limited lifetimes, however, based on the number and magnitude of the voltage kickbacks and possibly other factors. It may be desirable to reduce the magnitude of the kickback voltage that is absorbed by the TVSSs.
In addition to the above disadvantages, it may also be desirable to monitor the health of the load. For example, the capacitance of a load may decrease during its lifetime operation. If the capacitance falls below a baseline value, it is possible that the load will fail and/or will not achieve the expected performance. Power controllers that are capable of monitoring this load in addition to providing protection from in-rush current may be desirable.
Accordingly, a need exists for a power controller that has an improved inrush limiting circuit. A need also exists for a power controller that reduces the kickback voltage. A need also exists for a power controller that is capable of monitoring the health of the load. Embodiments described herein may address each of these needs or only one or two of these needs.